Learn Farsi: آب این چشمه امن و نوشیدنی است. - The water of this spring is safe and drinkable.

It is formed by taking the full infinitive verb نوشیدن (to drink) and adding the suffix ی. This creates an adjective showing capability or worth, meaning "capable of being drunk" or "drinkable".

I've seen نوشیدنی used to mean "a beverage". How can I tell it's an adjective here?

You are right! نوشیدنی can be a noun (a drink) or an adjective (drinkable). Here, it is linked by و to another adjective, امن (safe), and describes the water. The context makes it clear we are saying the water "is safe and drinkable" rather than "is safe and a beverage".

Where is the Ezafe in the phrase آب این چشمه?

The Ezafe is on آب, pronounced as "ab-e in cheshmeh". Notice that there is no Ezafe between این and چشمه. Demonstrative words like این (this) and آن (that) come right before their noun and never take an Ezafe.

In casual speech, how would امن و نوشیدنی be pronounced?

Instead of the formal "amn va nushidani", native speakers will blend the "and" into the phrase as a short "o" sound, pronouncing it "amn-o nushidani".